Some of these data were reported earlier and a manuscript is in preparation. In brief, groups of chimpanzees were immunized with either a formalin inactivated OSU 1-A vaccine that protected 66% of vaccinees against infection in human field trial studies or our experimental acellular vaccine which contained attachment, ciliotoxic and leukocyte recruitment activities and had been shown to protect hamsters against infection on challenge. Two chimpanzees that served as controls received plain broth in place of vaccine, and two additional chimpanzees which had recovered from the M. pneumoniae pneumonias nine months earlier were included in the study to determine whether prior infection would protect against subsequent challenge. Neither the OSU nor extract vaccine elicited an appreciable serum metabolism inhibition antibody response prior to challenge. Although the immunized chimpanzees became colonized, they did not produce the severe clinical signs of disease seen in the control animals. In contrast, the two chimps who had recovered from a prior infection were completely protected against colonization and disease, providing convincing evidence that immunization is a viable approach for protection against M. pneumoniae disease when the protective immunogens are presented to the host by an effective route of immunization.